Turkish President Recep Tayyip Erdogan has said on Friday that he is hoping that the United States will display reason when considering whether to impose sanctions following Turkey’s purchase of the Russian S-400 missile systems.

"We can begin actively using S-400 in April 2020 after their deployment and our specialists’ training are completed. I am hoping for the United States’ common sense," the NTV channel quoted him as saying, TASS reports.

Earlier on Friday, US Secretary of State Mike Pompeo announced that Washington had informed Ankara that the US believed the activation of the S-400 systems was unacceptable.